The invention relates to apparatus and methods for transferring semiconductor wafers between various wafer carriers, such as fifty-wafer quartz diffusion boats and twenty-five wafer plastic carriers, wherein the spacings between wafers in the different carriers are different; the invention relates more particularly to wafer transferring apparatus and methods that minimize the production of "silicon dust" particles broken from the edges of the semiconductor wafers during wafer transferring operations.
It is well known that numerous cleaning, coating and diffusion operations are required in the fabrication of semiconductor wafers, and that often fifty-wafer quartz boats are used, wherein there is three/thirty-seconds (3/32) of an inch center-to-center spacing between the wafers for diffusion operations, whereas for most other cleaning operations, plastic twenty-five wafer carriers, such as one manufactured by Flower, Inc. of Jaska, Minn., have three sixteenths (3/16) of an inch center-to-center spacing between wafers. The plastic wafer carriers also have long vertical side grooves or wafers guides, the bottoxsof which are approximately one hundred mils wide and the sides of which are tapered outwardly by approximately two degrees. A number of prior devices and methods for transferring wafers from the quartz diffusion boats to plastic carriers are known. The most elementary method is manual transfer by using tweezers to grip the edges of the wafers. This results in an excessive amount of breakage of silicon wafers, and also results in breaking off of minute pieces of silicon semiconductor material. Another prior art device is the one described in my U.S. Pat. No. 3,949,891. Several other automatic wafer transfer systems have been marketed by the present assignee and by other competitive companies. All of the prior wafer transfer systems generate a higher quantity of "silicon dust" that is not desirable. The most efficient, highest yield semiconductor manufacturing facilities include expensive equipment and require extensive precautions on the part of workers to maintain the wafer fabrication environment at the highest possible level of purity. For example, expensive laminar air flow systems, and expensive dust filters often are used. The most modern facilities completely eliminate use of tweezers and manual handling of wafers in order to reduce the amount of silicon dust that is produced, and all workers wear clean gowns, hair nets, and in some cases, even face masks. However, due to build-up of static electricity on various wafer handling components, even small amounts of silicon dust can become attached to wafer surfaces or surfaces that touch wafers. For example, silicon dust can become attached to the sides of the grooves of the above-mentioned plastic wafer carriers, and when the wafers are transferred into such carrier, the peripheral portion of the active face of the wafer may slide against such silicon dust, causing defects in one or quite a few peripheral circuits on the wafer. This, of course, reduces yield and increases the overall cost per unit of the circuits being manufactured.
Another problem with previous automatic wafer transfer systems is that they have required too much floor space. Available room in a wafer fabrication facility is scarce, due to the very high cost per square foot of maintaining an ultra-pure environment. Previous wafer transfer devices have operated too slowly, some of them having a transfer cycle time of as long as twenty (20) minutes to transfer fifty wafers from one type of carrier to another.
Thus, there remains a need for a relatively high speed, low cost, wafer transfer system that greatly reduces the amount of physical contact with the wafers over previous wafer transfer systems in order to greatly reduce or eliminate the production of silicon dust.
Some of the previous wafer transfer systems not only produce quite a lot of silicon dust as the edges of the wafers contact various holding elements during the wafer transfer process, they produce dust at such a location that it falls directly onto a region in which other silicon wafers are presently positioned. Some of this dust inevitably comes into sliding contact with the surface of some of the wafers, causing yield-reducing defects therein.
Therefore, it is an object of the invention to provide an improved wafer transferring apparatus and method which greatly reduces or eliminates sliding contact between the active surface of a wafer, and furthermore eliminates production of any silicon dust at a location above any other semiconductor wafers.
It is another object of the invention to produce an improved wafer transfer system that eliminates production of silicon dust and can be implemented in a small amount of space.